In chambers used for chemical vapor deposition (CVD), physical vapor deposition (PVD), etching, rapid thermal processing (RTP), or similar processes, in which substrates such as semiconductor wafers, flat panel displays, etc., are processed, it is common to support the substrate on a substrate support such as an electrostatic chuck. The temperature of the substrate may be monitored and controlled during processing.
Optical temperature measurement techniques can be used to monitor real-time temperature of the substrate during processing. One technique of monitoring temperature involves measuring the infrared (IR) energy emitted from the surface of a heated substrate, then converting this measured energy into a temperature reading.
A scanning monochrometer and lock-in amplifier manufactured by Thermionics Northwest, Inc. as model DRS 1000.TM. has been used to measure reflected light from a wafer. The reflected light varies in spectrum with the temperature of the wafer, thereby permitting the temperature of the wafer to be determined from this measurement. In order to permit the monochrometer to accurately scan over a range of reflected wavelengths, a series of filters are rotated within the monochrometer. This method, and its associated apparatus, are bulky, expensive, and slow.
U.S. Pat. No. 4,956,538 to Moslehi, the disclosure of which is hereby incorporated by reference, discloses another technique wherein laser energy is used to determine the real-time emissivity of the substrate based on a measurement of reflectance, and optionally reflectance in combination with transmittance. This real-time emissivity is then used to calibrate a pair of pyrometers, from which a temperature measurement is derived. The arrangement of Moslehi includes a complex system of mirrors and light pipes for purposes of accurately measuring light reflected off the surface of a semiconductor substrate.
Various problems are known to exist with such techniques which can adversely affect accuracy. First, such techniques are very sensitive to the optical properties of the substrate surface (sometimes referred to as emissivity). Second, light that originates from other areas within the processing chamber, such as a plasma field, acts as "noise" which adversely affects the accuracy of the measurement. Third, substrates often become transparent or semi-transparent during normal processing temperatures. When a photodetector is located above the substrate to measure reflected light, the detector looks "through" the substrate thereby adversely affecting the accuracy of the measurement. Fourth, the "window" through which the light source transmits light into the processing chamber can become obscured by the materials within the processing chamber which can attach themselves to the surface of the window.
It has proven difficult to accurately measure substrate temperatures during processing and, accordingly, temperature control suffers. Therefore, it is desirable to provide a method and apparatus for reliably monitoring substrate temperature during processing.